NASA Discoveries: Neutrinos and Cosmic Fog

1. Mar 10, 2008

SF

A NASA satellite has uncovered evidence that a sea of neutrinos, almost weightless elementary particles that zip around at nearly the speed of light, permeates the universe. The discovery is part of a treasure trove of findings gleaned from data collected by the Wilkinson Anisotropy Probe (WMAP). This includes clear evidence the first stars took more than a half billion years to create a cosmic fog and a far better understanding of the burst of expansion in the universe's fraction of a second.

http://www.princeton.edu/main/news/archive/S20/49/27E85/index.xml?section=science

2. Mar 10, 2008

nesp

From the article:

"The WMAP results also show that the universe is growing colder and darker. Atoms now make up 4.6 percent of the current universe, dark matter makes up 23 percent and dark energy makes up 72 percent, according to its measurements. The microwaves that WMAP sees come from when the universe was only 380,000 years old, when atoms made up 12 percent of the universe, dark matter made up 63 percent, photons made up 15 percent, neutrinos made up 10 percent and the dark energy was negligible."

So we have
Now:
Atoms 4.6
DM 23.0
DE 72.0
Total 99.6
0.4 photons+neutrinos?

Then:
Atoms 12.0
DM: 63.0
Photons: 15.0
Neutrinos: 10.0
DE: ~0
Total: 100.0

I assume the atoms were converted to energy in nuclear reactions. But where did the neutrinos and dark matter go? And where did the dark energy come from? Does normal matter/energy convert to dark matter/energy?

3. Mar 10, 2008

Wallace

Those figures related to the fractional density of each component, so to understand them you need to think about how energy densities evolve as the Universe expands.

First think about matter. As the Universe expands the volume increases but there is no more matter produced so the density of matter drops as

$$\rho_m \sim \frac{1}{V}$$

the volume can be expressed as the cube of the scale factor a giving

$$\rho_m = \rho_{m0} a^{-3}$$

where $$\rho_0$$ is the density at some initial time. This applies to dark and normal matter. Some matter is obviously converted to energy in stars, but this is a very smaller fraction of the total matter and can be ignored.

Now treat radiation, the number of CMB photons stays the same but their density drops the same way as matter. In addition, the energy of each photon drops as ~1/a due to redshift leaving

$$\rho_r = \rho_{r0} a^{-4}$$

Now, the last remaining component is dark energy. If dark energy is a cosmological constant or vacuum energy (which the data says it must be very similar to) then the density always remains the same.

What this all means is that initially the density of say matter is much more than dark energy, but as the Universe expands the density of matter drops while dark energy remains the same, leading to dark energy eventually dominating which is what we see today.

Does that make sense?

Last edited: Mar 10, 2008
4. Mar 10, 2008

nesp

Very much so, thanks for the explanation. The key to the figures, then, is the expansion of the Universe and dark energy being a cosmological constant. The rest is explained from those two assumptions, as per your equations. Thanks much!

5. May 18, 2010

Dr.Prashant k

Good morning sir,
I am not a rocket scientist,neither physicist nor mathematician,i am just a dentist who scored just a good marks in physics,chemistry,maths and biology during my school days,but despite of my profession some part of me is forcing me to this field.
As my concern is not about this issue for now i was just wondering can "Dark matter be converted into energy".
Please do reply to boost my will so that i can think more for our future and for humanity.

Regards,